The invention relates to a method of determining a target spatial coordinate using an apparatus comprising a movable hand-held probe, having a pointing element for pointing the target spatial coordinate, and a portable base unit provided with line-of-sight distance coupling means, wherein the hand-held probe is coupled to the base unit by means of the line-of-sight coupling means via an attachment point, and wherein the base unit is provided with sensors providing measuring signals for measuring length or a change in length of the line-of-sight distance coupling means and rotation of the line-of-sight distance coupling means in at least one degree of freedom, and computer-controlled processing means communicatively connected to the sensors for processing measuring signals delivered by the sensors.
An apparatus of the above mentioned type is known from EP patent 1,226,401, which can be used for measuring the shape or contour of two-dimensional or three-dimensional objects, such as small objects to be placed on a measuring table, or relatively large objects disposed in a room.
The known apparatus is equipped with a cord or a wire, for connecting the movable hand-held probe to the portable base unit via an elongated arm provided with the base unit. Therefore, only a single sensor suffices for determining the length or change in length of the cord or the wire.
A second sensor in the apparatus is used in order to enable accurate determination of the angle or angular displacement of the cord caused by a change in position of the measuring probe. The second sensor is arranged to measure the rotational displacement of the arm in, for example two dimensions or three dimensions.
In an example, the sensors for measuring length or a change in length of the cord or the wire and rotation of the arm in at least one degree of freedom are understood to be sensors in the form of pulse generators, wherein the number of pulses delivered during use is proportional to a change in length or angular displacement of the cord or the wire or of the arm coupled thereto. Another option is to use potentiometers for measuring the rotation of the arm, and the change in length of the cord or the wire.
The computer-controlled processing means are arranged for processing the measuring signals delivered by the sensors, i.e. the angle or angular displacement of the line-of-sight distance coupling means or change in length of the line-of-sight distance coupling means, into position data of the hand-held probe.
EP patent 1,226,401 provides an example in which the coordinates of the attachment point and the target spatial coordinate are the same because the cord or wire is attached to the end of the pointing element, which pointing element is used for pointing the target spatial coordinate.
However, in such an example, a problem arises in case a spatial coordinate is to be measured which is not in direct line-of-sight with the base unit. In order to accurately determine the spatial coordinate, the cord or wire needs to be directly spanned in a straight line between the base unit and the hand-held probe without obstacles or obstructions. In case the spatial coordinate is not in direct line-of-sight with the base unit, the computer-controlled processing means will unavoidably include inaccuracies in the determination of the spatial coordinate. The above is, for example, important for measuring a target spatial coordinate behind an object.
The inventors of the present invention further noticed that in practice situations the target spatial coordinate can not always be directly, physically pointed at, due to obstacles, obstructions, narrow passages, or the like. In such a case, the apparatus of EP 1,226,401 will introduce offset inaccuracies as the intended spatial coordinate, i.e. the target spatial coordinate, does not equal the attachment point of the line-of-sight distance coupling means, i.e. a cord or a wire.
Consequently it is an object of the invention to provide for an improved method and apparatus for accurately determining a target spatial coordinate in more versatile situations.